1. Field of the Invention
The present invention relates to a magnetoresistive effect element and a magnetic memory having the same.
2. Related Art
A magnetoresistive effect element having magnetic films is used for magnetic heads, magnetic sensors and so forth, and proposed to be used for solid magnetic memories. In particular, there is an increasing interest in a magnetic random access memory (which will be also hereinafter referred to as MRAM), which utilizes the magnetoresistive effect of a ferromagnetic substance, as a next generation solid nonvolatile memory capable of carrying out a rapid reading/writing and an operation with large capacity and low power consumption.
In recent years, as a magnetoresistive effect element which has a sandwich structure having one dielectric layer between two magnetic metal layers and which allows a current to pass in a direction perpendicular to the plane of the film and utilizes a tunnel current, a so-called “ferromagnetic tunneling magnetoresistive effect element (which will be also hereinafter referred to as a TMR element)” has been proposed. Since the rate of change in magnetoresistive effect of a ferromagnetic tunneling magnetoresistive effect element reaches 20% or more (see J. Appl. Phys. 79, 4724 (1996)), the possibility that it is applied to MRAMs is enhanced.
This ferromagnetic tunneling magnetoresistive effect element can be realized by depositing a thin Al (aluminum) layer having a thickness of 0.6 to 2.0 nm on a ferromagnetic electrode, and thereafter, exposing the surface thereof to oxygen glow discharge or oxygen gas to form a tunnel barrier layer of Al2O3.
In addition, there has been proposed a ferromagnetic single tunnel junction having a structure wherein an antiferromagnetic layer is provided so as to contact one ferromagnetic layer forming the ferromagnetic single tunnel junction, so that the exchnage coupling force causes the magnetization inversion of the one ferromagnetic layer to be difficult to occur to form a magnetization fixed layer in which the direction of magnetization is fixed (see Japanese Patent Laid-Open No. 10-4227).
In addition, a ferromagnetic tunnel junction using magnetic particles dispersed in a dielectric, and a ferromagnetic double tunnel junction (continuous layer) have been proposed (Phys. Rev. B58 (10), R5747 (1997), Journal of Applied Magnetic Society 23, 4-2, (1999), Appl. Phys. Lett. 73 (19), 2829 (1998), Jpn. J. Appl. Phys. 39, L1035 (2001)).
Also in these cases, the rate of change in magnetic resistance can be in the range of 20% to 50%, and the decrease of the rate of change in magnetic resistance can be suppressed even if a voltage value applied to the ferromagnetic tunneling magnetoresistive effect element is increased in order to obtain a desired output voltage, so that there is some possibility that they are applied to MRAMs.
If a TMR element is used for an MRAM, a magnetization fixed layer, in which the direction of magnetization of one of two ferromagnetic layers sandwiching a tunnel barrier layer therebetween is fixed, serves as a magnetization reference layer, and a magnetization free layer, in which the direction of magnetization of the other ferromagnetic layer is easy to be inverted, serves as a storage layer. Information can be stored by causing states, in which the directions of magnetization in the reference layer and storage layer are parallel and antiparallel to each other, to correspond to binary information “0” and “1”.
Information to be recorded is written by inverting the direction of magnetization in the storage layer by an induction field which is caused by a current passing through a writing line provided in the vicinity of a TMR element. The recorded information is read by detecting a variation of magnetic resistance due to the TRM effect.
In order to fix the direction of magnetization in the reference layer, there is used a method wherein an antiferromagnetic layer is provided so as to contact a ferromagnetic layer so that it is difficult to invert magnetization by the exchnage coupling force. Such a structure is called a spin-valve structure. In this structure, the direction of magnetization in the reference layer is determined by carrying out a heat treatment while applying a magnetic field (a magnetization fixing annealing). On the other hand, the storage layer is formed so that the magnetization easy direction is substantially the same as the direction of magnetization in the reference layer by applying magnetic anisotropy.
The magnetic storage element using the ferromagnetic single tunnel junction or ferromagnetic double tunnel junction has a potential that it is nonvolatile, the writing/reading time is a short time of 10 nanoseconds or less, and the number of capable rewriting operations is 1015 or more. In particular, the magnetic storage element using the ferromagnetic double tunnel junction can decrease the rate of change in magnetic resistance even if the voltage applied to the ferromagnetic tunneling magnetoresistive effect element to obtain a desired output voltage value is increased, so that it can obtain a high output voltage to have preferred characteristics as a magnetic storage element.
However, with respect to the cell size of the memory, if one transistor-one TMR architecture having a memory cell comprising one transistor and one TMR element (see, e.g. U.S. Pat. No. 5,734,605) is used, there is a problem in that the size can not be smaller than that of a DRAM (Dynamic Random Access Memory) of a semiconductor.
In order to solve this problem, there have been proposed a diode type architecture wherein a TMR element and a diode are connected in series between a bit line and a word line (see U.S. Pat. No. 5,640,343), and a simple matrix type architecture wherein a TMR element is arranged between a bit line and word line (see DE19744095, WO9914760).
However, if the memory capacity is increased and the size of the TMR element is scaled down, there is a problem in that thermal fluctuation occurs, and there is some possibility that spin information disappears. In addition, there is a problem in that the switching magnetic field increases due to the decrease of the size of the TMR element.
The coercive force, i.e. the switching magnetic field, depends on the size and shape of the element, magnetization of the ferromagnetic material, thickness and so forth. In general, if the size of the storage element increases, the switching magnetic field increases. This means that, if the TMR element having the tunnel junction is used for an MRAM as a storage element, a large magnetic field due to current is required, and power consumption increases. Moreover, if high integration is considered, there is a serious problem in that the increase of power consumption is more remarkable.
In addition, there is a problem in long-term thermostability under the influence of diffusion of Mn atom or the like due to heat.
As described above, in order to realize amass storage of a magnetic memory, there is required a magnetoresistive effect element, which has a large MR ratio, a small switching magnetic field and a superior thermostability even if the size of a TMR element is decreased, and a magnetic memory using the same.